The present invention relates to a simultaneous double side polishing and processing method for a substrate and processing apparatus, a polishing and processing method for a magnetic disk uses the same, a manufacturing method for a magnetic disk, and a magnetic disk. The present invention relates to a simultaneous double side polishing and processing method without polishing sags in the peripheral portion of a substrate, and particularly to a substrate processing method and processing apparatus suitable for manufacturing a high density magnetic disk that is superior in sliding characteristics such as head flying characteristic, CSS (Contact-Start-Stop) characteristic, etc. and having a high reliance.
The surface polishing of a plate-like substrate is a fundamental technique of polishing, which has been heretofore studied with respect to substrates for various applications. Here, a magnetic disk device used for an external memory of a computer will be described below as a typical example.
As a substrate for a magnetic disk formed from a magnetic film formed by use of a film forming technique such as sputtering, plating, ion plating or the like, there has been used a substrate formed by applying a nickel-phosphorus (Ni-P) plating having a thickness of scores of .mu.m as the undercoating film for the formation of a magnetic film to the surface of an aluminum alloy substrate, and applying double side grinding or double side lapping as shown as a process illustrates in FIG. 7 to said surface and further applying double side polishing processing using abrasive grains of various grain sizes thereto to finish the surface having the roughness in the range of 0.001 to 0.003 .mu.m Ra, 0.005 to 0.020 .mu.mR max. Thereafter, in order to avoid adhesion of a head at the time of the CSS drive as a magnetic disk device, a peripheral polishing process called texture process is applied to the surface to readjust the surface roughness to have 0.004 to 0.008 .mu.m Ra.
The texture formed in the periphery of the substrate avoids the adhesion of the head as described above. In addition, this texture improves the magnetic characteristic of a magnetic film formed on the surface as discussed, for example, in "International Magnetics Conference (1987) GA02, GA03".
The magnetic disk is manufactured by the film forming processes employing for example a magnetic film, a protective film on the substrate in the processes as described above and further is post-processes to from a lubricating film.
FIG. 8 is a partly cutaway perspective view of a magnetic disk device, in which a magnetic disk 1 is combined with a magnetic head 2 as shown to complete a magnetic disk device.
FIG. 9 is a schematic view showing a mode in which a magnetic head 2 flies on a rotating magnetic disk 1. In order to increase a recording capacity of the magnetic disk device, a flying height H.sub.fly of the magnetic head is very small, and the required height is from 0.2 .mu.m or less to 0.1 .mu.m or less. Particularly, the size of a fine projection Rp on the surface of the magnetic disk gives rise to a problem in the head's flying characteristic. The disk surface has been subjected to processing using a polishing tape or a head to decrease the fine projection. The surface roughness subjected to texture processing is less than 0.004 .mu.mRa to a state close to the surface roughness subjected to polishing process.
The disk surface on which is conducted CSS characteristic test at a very low flying height has scratches resulting from polishing of a substrate. That is, there is a sliding defect resulting from a fine raised portion of a groove shoulder portion of the scratch. When the defect is large, a head crush results. From the foregoing, with a magnetic disk for high density recording, it is necessary to further decrease the head flying height. To this end, it is necessary to further decrease the surface roughness of the polished surface of the magnetic disk substrate so as to have the surface less than 0.001 .mu.m Ra and 5 nm (0.005 .mu.m) Rmax.
Furthermore, the present inventors found that in order to decrease the head flying height and to fly the magnetic head in a stabilized manner, it is important to provide the surface roughness with higher precision and increase a plane degree representative of a surface shape of the magnetic disk. Particularly, in order to shorten the access time of the magnetic disk device, the magnetic disk is rotated at the high speed, i.e., 5000 or 7000 r/min or more as compared with 3600 r/min of prior art. As the dynamic shape precision of the disk surface is determined in order that the magnetic head is files in a stabilized manner with respect to the rotation of the disk, data of runout and must be considered acceleration. It has been found to as shown in FIG. 10, that the acceleration extremely rapidly increases in a disk of the same plane degree in accordance with the number of revolutions of disk.
Therefore, it is essential to improve the plane degree in order to achieve the stabilized head flying characteristic even at the high speed rotation. With respect to the shape precision of the substrate for the magnetic disk obtained a conventional polishing process, for example, in a typical Ni-P plated aluminum alloy substrate, the plane degree is 6.8 .mu.m and standard deviation is 2.2 .mu.m; and in a partly applied glass plate, the plane degree is 5.1 .mu.m, and standard deviation is 1.3 .mu.m. With respect to the flying characteristic of the magnetic disk using the aforementioned substrates, the head flying height is approximately 0.08 to 0.1 .mu.m. It has been extremely difficult to obtain stabilized a head flying height of 0.08 .mu.m or less.
Further, when the speed of desk is 3600 r/min the runout and acceleration representative of the dynamic surface precision of the magnetic disk are 1 to 15 .mu.m and 1 to 5 m/s.sup.2, respectively. From the foregoing, in order to decrease the flying height of the magnetic head and realize the magnetic disk corresponding to high recording density and excellent in sliding characteristics such as CSS characteristic, it is necessary to have a high precision substrate formed by a new process method different from a conventional process.
A typical one side polishing method for a disk substrate will be described with reference to FIG. 11. In the conventional one side polishing, as shown, a work (a substrate) 3 is adhered to a holder 13 with resin or the like, and one side of the work 3 is polished, after which the work 3 is disengaged from the holder 13 and then washed. Thereafter, the substrate is inverted and adhered to the holder, and the other side is polished in a manner similar to the former. Therefore, the processed surface is deformed by a strain caused by adhesion and an uneven strain resulting from processing of surface and back of the work surface to make it difficult to obtain a high precision plane degree. Moreover, the polisher 14 brings forth degradation resulting from a conventional mirror polishing, by polishing with the polisher 14 a sag tends to occur in a peripheral portion of the work due to viscoelastic deformation of the polisher caused by pressurization to make it difficult to obtain a high precision plane degree.
In addition to the aforementioned polishing method, as a method for separating a rotating surface plate from a substrate surface, causes abrasive grains to flow into a gap therebetween. Additionally by performing polishing in a non-contact state, a processing method called a float polishing has been known. This method can overcome the sag of the plane degree in the peripheral edge of the substrate thus overcoming the which has been defect of the conventional polishing, and float polishing can attain the surface roughness of a degree of an angstrom.
However, the greatest problem of this float polishing method is that one surface of a substrate is polished while the surface is adhered and fixed to a base, and therefore, double sides cannot be simultaneously processed and processing this float polishing method is limited to one side. Accordingly, this process can't be mass produced. Furthermore, when the substrate is inverted to process the other surface, the substrate is once again removed from the base, and the already polished flat surface is again adhered and fixed to the base. Therefore, particular care in handling so as not to produce a scratch is required impairing the automation of polishing process. Moreover, the simultaneous double side processing employing the float polishing has been said to be impossible in terms of technique and has been disregarded.
As previously mentioned, to attention given to a fine projection on the disk surface for stabilizing the flying characteristic and CSS characteristic of the magnetic head, the substrate having a conventional surface roughness and the decrease in fine projection is insufficient to deal with the stabilization of flying less than 0.08 .mu.m of flying height. In this connection, it is necessary that the surface roughness of the substrate be less than 1 nm Ra, and the substrate is less than a few nmRmax. It is further required that the plane degree which is the surface shape of the magnetic disk be less than 1 .mu.m, and the runout and acceleration are less than 1 .mu.m and less than 1 m/s.sup.2, respectively. In order to obtain such a magnetic disk, it is necessary to considerably improve the surface precision of the plane polishing of the substrate.